If you have a coworker who doesn't understand it, pair them with someone more senior until they get up to speed. If they are simply cognitively incapable of understanding the concept of mapping a function, in my opinion they are not capable of becoming a good developer.
Yes, there are times when reducing lines of code reduces clarity. But in my experience they are rare and exceptional. Far more real world code lacks clarity due to verbosity than from being overly terse.
Sure, they could be taught. But they're busy being taught other things - Android, say, or the latest JS framework. So there's an opportunity cost to teaching them - it takes away time and attention from teaching them other things, things that may move us closer to our business goals. So in practice, yes we can teach them, and no we won't.
And now that one line of code can decrease clarity for the team.
In my opinion and experience, Resharper is the easiest way to learn LINQ, before you even start looking at all the other dumb, inconsistent and error-prone stuff that people will do because it compiles, blissfully unaware of all the foot-guns it contains.
The problem is, clarity for who? For the developer? For another, equally skilled developer? Or for the developer's less skilled coworkers? For the least skillful coworker? To whom does the code need to be clear?